Climate change is one of the greatest environmental challenges. The increasing concentration of carbon dioxide in the atmosphere is to a very large part due to global warming. The CO2 from human activity is essentially discharged into the atmosphere through the combustion of fossil fuels in power stations.
To combat CO2 emissions, one technology is aimed at capturing the CO2 emitted during the combustion of carbon fuels in order to sequester it underground. One of the constraints posed is how to separate the CO2 from the flue gas in which its fraction conventionally does not exceed 15%, but which entails substantial energy to carry out the separation.
One option consists in separating the nitrogen from the air upstream of the combustion, almost only CO2, water and combustion products then remaining at the outlet of the boiler. The boiler therefore operates in oxyfuel combustion mode. A portion of the flue gas (essentially CO2) may be recycled with oxygen in order to prevent excessively high temperatures being reached in the boiler. CO2 capture is therefore provided at lower cost.
This technique is promising, both from the investment standpoint and the overall energy efficiency.
As long as infrastructures for channeling and sequestering the CO2 are not close enough, or as long as the price per ton of CO2 sold is not high enough, it cannot be economically profitable to capture all the CO2 emitted by a power station.
One solution would be to employ partial CO2 capture. However, partial CO2 is not well suited to oxyfuel combustion technology. In effect, it is necessary to operate in 100% oxyfuel combustion mode or 100% in air mode, but it is difficult to move away from these regimes. This is because if there is more than 30% nitrogen in the flue gas, CO2 separation loses all the advantages that are obtained when the flow is more concentrated.
Thus, the reference solution for partial capture would be to invest 100% in an ASU (air separation unit) and to operate this at 100% of its capacity. However, it is possible to invest only partly in a compression/drying unit (or invest 100% in it but to operate it only with a level of CO2 that it is desired to capture). Unfortunately, this compression/drying unit only represents a small part of the investment and energy consumed thereby, unlike an ASU.
Moreover, operating with an ASU at 100% of its capacity means consuming an amount of energy which is constant over time. This precludes adapting the operation to the variations in available energy cost and flow.
From this starting point, one problem that arises is how to provide a combustion process suitable for partial CO2 capture and for variable energy supply.